1. Field of the Invention
The present invention relates to a digital TV system, and more particularly to a method and apparatus for compensating channel distortion in the digital TV system.
2. Discussion of Related Art
Generally, a digital TV system removes ghost signals generated in a channel using the equalizer. Specifically, in the digital TV system, the received signal includes a training sequence signal in a field synchronous section of each field to help a receiver perform equalization. The receiver compensates for distortion generated in the channel using the training sequence. FIG. 1 shows the structure of one frame of a conventional digital TV signal and FIG. 2 shows a structure of the field synchronizing signal of FIG. 1.
Referring to FIG. 1, a frame includes two fields. Each field has 313 data segments where one including one field synchronous segment containing the training sequence signal and 312 general data segments. Also, each data segment consists of a 4 symbol data segment synchronizing signal and 828-symbol data. FIG. 2 shows a field synchronizing signal of one data segment length including a data segment synchronizing pattern in the first four symbols, pseudo random sequences of PN 511, PN 63, PN 63 and PN 63 in the following symbols, and information related with the VSB mode is in the next symbols. The polarity of the second of the three PN 63 sections alternates. That is, the polarity changes from xe2x80x981xe2x80x99 to xe2x80x980xe2x80x99 and from xe2x80x980xe2x80x99 to xe2x80x981xe2x80x99. Accordingly, even and odd fields are determined according to the polarity of the second PN 63.
FIG. 3 shows modelling of a conventional channel having no moving ghost. When there is no moving ghost in the channel, a high definition TV (HDTV) receiver receives an original signal and ghost signal, as shown in FIG. 3. This ghost signal loaded in the channel is removed by the equalizer of the receiver using the training sequence signal in the field synchronizing signal. When a moving ghost signal does not exist in the channel, the level of the received signal as seen by the HDTV is almost always uniform. Thus, the HDTV receiver is able to remove the ghost using only the training sequence in each field synchronizing section.
However, when a moving ghost is loaded on the channel, the training sequence is not sufficient to remove a moving ghost because the state of the received signal would changes constantly. For example, a moving ghost generated due to an airplane, as shown in FIG. 4, varies in sequence of 1xe2x86x922xe2x86x923. Accordingly, the conventional method of removing the ghost existing in the channel using only the training sequence cannot effectively cope with the moving ghost in the channel, resulting in a deterioration in the performance of the system.
Accordingly, an object of the present invention is to solve at least the problems and disadvantages of the related art.
An object of the present invention is to provide a method and apparatus for compensating channel distortion by detecting and removing a moving ghost included in a received signal.
Another object of the present invention is to provide a method and apparatus for compensating channel distortion performing equalization using a data portion as well as a training sequence when a received signal has a moving ghost, to thereby effectively cope with a case where the ghost changes quickly in a channel.
A further object of the present invention is to provide a method and apparatus which compensates for channel distortion by detecting and removing a moving ghost that moves slowly.
A still further object of the present invention is to provide a method and apparatus which compensates for channel distortion by performing equalization in a blind mode when it is possible for an equalizer to diverge, to thereby carry out stable equalization.
A still further object of the present invention is to provide a method and apparatus which compensates for channel distortion by resetting a channel decoder including the equalizer to start from the beginning when the equalizer is already diverged, to thereby perform stable equalization.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.
To achieve the objects and in accordance with the purposes of the invention, as embodied and broadly described herein, a method for compensating channel distortion comprises calculating a DC value from input data, reading the DC value a predetermined number of times in a specific cycle, detecting maximum and minimum values from the read DC values, judging if there is a moving ghost in the channel from the difference between the maximum and minimum values, and judging if three is a possibility that the equalizer diverges from the MSE of a signal equalized by the equalizer; performing the equalization in the training sequence mode if it is judged that the moving ghost does not exist in the channel and if there is no possibility that the equalizer would diverge; carrying out the equalization in the blind mode if it is judged that the moving ghost does not exist in the channel but there is a possibility that the equalizer would diverge; executing the equalization in the data mode if it is judged that the moving ghost exists in the channel and there is no possibility that the equalizer would diverges; and performing the equalization in the data mode and blind mode when it is judged that the moving ghost exists in the channel and there is a possibility that the equalizer would diverges.
Particularly, the present invention includes judging that a moving ghost does not exist in the channel if the difference between the maximum and minimum values of the DC values, which are read a predetermined number of times in a specific cycle, is smaller than a predetermined first critical value; cancelling the data mode when it is judged that there is no moving ghost in the channel; and deciding that there is no possibility that the equalizer would diverge when it is judged that the MSE value of the signal equalized by the equalizer is smaller than a predetermined second critical value, to operate the equalizer in the training sequence mode.
The present invention also comprises deciding that there is no moving ghost in the channel when it is judged that the difference between the maximum and minimum values of the DC values, read a predetermined number of times in a specific cycle, is smaller than the predetermined first critical value; cancelling the data mode when it is judged that there is no moving ghost in the channel; and deciding that there is a possibility that the equalizer would diverge when it is judged that the MSE value of the signal equalizer by the equalizer is larger than the predetermined second critical second critical value, to operate the equalizer in the blind mode.
The present invention further comprises deciding that the moving ghost exists in the channel when it is judged that the difference between the maximum and minimum values of the DC values, read a predetermined number of times in a specific cycle, is larger than the predetermined first critical value; turning on the data mode when the moving ghost exists in the channel and the VSB mode of the input data corresponds to a mode for terrestrial broadcasting; and deciding that there is no possibility that the equalizer would diverge when it is judged that the MSE value of the signal equalized by the equalizer is smaller than the predetermined second critical value, to operate the equalizer in the data mode.
The present invention further comprises deciding that the moving ghost exists in the channel when it is judged that the difference between the maximum and minimum values of the DC values, read a predetermined number of times in a specific cycle, is large than the predetermined first critical value; turning on the data mode when the moving ghost exists in the channel and the VSB mode of the input data corresponds to a mode for terrestrial broadcasting; and deciding that there is a possibility that the equalizer would diverge when it is judged that the MSE value of the signal equalized by the equalizer is larger than the predetermined second critical value, to operate the equalizer in the blind mode.
In another embodiment, a method for compensating channel distortion according to the present invention comprises calculating a DC value from input data, reading the DC value a predetermined number of times in a specific cycle, detecting maximum and minimum values from the read DC values and storing them; shifting each values of a plurality of memories to the next memory and storing the minimum value in the memory which is finally left; judging if the moving ghost exists in the channel using the difference between the stored maximum and minimum values and the difference between the minimum value stored in the memory and current maximum value, and judging if there is a possibility that the equalizer would diverge from the MSE value of a signal equalized by the equalizer; performing the equalization in the training sequence mode when it is judged that the moving ghost does not exist in the channel and there is no possibility that the equalizer would diverge; carrying out the equalization in the blind mode when it is judged that the moving ghost does not exist in the channel but there is a possibility that the equalizer would diverge; executing the equalization in the data mode when it is judged that the moving ghost exists in the channel and there is no possibility that the equalizer would diverge; and performing the equalization in the data mode and blind mode simultaneously when it is judged that the moving ghost exists in the channel and there is a possibility that the equalizer would diverge.
The judgement step comprises deciding if the difference between the maximum and minimum values is smaller than a predetermined first critical value; sequentially comparing the minimum values stored in the plurality of memories with a currently read maximum value and judging if the difference between them is larger than a predetermined third critical value when the difference between the maximum and minimum values is smaller than the predetermined first critical value; and judging that there exists the moving ghost in the channel when the difference between the maximum and minimum values is larger than the first critical value, or the difference between the value stored in each memory and the currently read maximum value is larger than the third critical value.
In still another embodiment of the method for compensating channel distortion according to the present invention comprises calculating a DC value from input data, reading the DC value a predetermined number of times in a specific cycle, detecting maximum and minimum values from the read DC values, calculating the average value of them and storing it; shifting each of values of a plurality of memories to the next memory and storing the average value in the memory which is finally left; judging if the moving ghost exists in the channel using the difference between the stored maximum and minimum values and the difference between the average value stored in the memory and current average value, and judging if there is a possibility that the equalizer would diverge from the MSE value of a signal equalized by the equalizer; performing the equalization in the training sequence mode when it is judged that the moving ghost does not exist in the channel and there is no possibility that the equalizer would diverge; carrying out the equalization in the blind mode when it is judged that the moving ghost does not exist in the channel but there is a possibility that the equalizer would diverge; executing the equalization in the data mode when it is judged that the moving ghost exists in the channel and there is no possibility that the equalizer would diverge; and performing the equalization in the data mode and blind mode simultaneously when it is judged that the moving ghost exists in the channel and there is a possibility that the equalizer would diverge.
An apparatus which compensates for channel distortion according to the present invention comprises an analog/digital converter for converting an input signal into digital data; a synchronizing signal detector for detecting synchronizing signals from the digital data; a VSB mode detector for detecting a VSB mode from the digital data; an input MSE calculator for calculating the MSE of the digital data; a DC calculator for calculating a DC value included in the digital data; an equalizer for removing a ghost included in the digital data in at least one of three modes of training sequence mode, data mode and blind mode; an output MSE calculator for calculating the MSE of data equalized by the equalizer; and a controller for determining the equalization mode of the equalizer using the operation results of the VSB mode detector, input MSE calculator, DC calculator and output MSE calculator, and generating a corresponding control signal.
The controller resets the equalizer when it is judged that the synchronizing signal detector did not detect the synchronizing signals, or the equalizer completely diverged.